Pilot operated diaphragm valves have had considerable success in both consumer and industrial hydraulic systems. For example, they have been used in mixing valve assemblies in the past for many years in clotheswashing machines for mixing predetermined proportions of hot and cold water to provide the appliance with the desired temperature water, as well as controlling the flow rate of the water supplied.
Typically, these mixing valves include two diaphragm operated valves, one for cold water and one for hot water with some type of control system for actuating either the hot or cold water valve, or for actuation of both valves to give a mix of the hot and cold fluids. These valves are very limited in mix temperature control due to effects of different inlet pressures of the hot and cold, water temperature variables, and different flow pressure drops through each section of the mixing valve itself. Thus, outlet mix temperature of these valves is solely dependent upon whatever the full flow hot and cold valves will provide for the given variables and, therefore, little control of actual mix temperature is achieved.
Other previous mixing valves did provide control of the hot and cold mix temperature through a mechanical sensor/actuator which provided a selectivity of mix temperatures achieved by proportioning the hot and cold fluids from the solenoid operated valves in a mechanically proportioning mixing chamber. In these mixing valves, the hot and cold pilot operated valves were also either fully "on" or "off" and depended upon the thermostatically controlled mixing chamber to achieve some desired degree of mix temperature.
In the past these valves have included pilot operated diaphragm valves with actuators that control on/off flow through a pilot passage extending through the diaphragm and the position of this actuator has been controlled by an electromagnet. These prior diaphragm operated valves, however, have been found only useful in positioning the main diaphragm valve in either a fully opened or fully closed position because attempts in attaining modulating capability of these valves at the pilot valve orifice has been unreliable since the pilot valve under these conditions has been found to have an inherent propensity to inadvertently close and cause premature main valve closure, especially upon transient current or voltage surges through the electromagnetic coil or because of transient pressure surges at the valve inlet.
One such pilot operated diaphragm valve is disclosed in the McCarty, Jr., et al., U.S. Pat. No. 3,672,627, and it includes a diaphragm operated main valve for controlling flow between an inlet and an outlet with one side of the diaphragm being exposed to inlet pressure and the opposite side of the diaphragm forming an intermediate chamber in the housing in which fluid pressure is controlled by an electromagnetic coil driven actuator reciprocal in the housing having a pilot seal at its inner end that selectively blocks flow through a central pilot passage in the main diaphragm valve.
With the electromagnetic coil de-energized, the actuator is biased by a coil spring to close this pilot passage and when fluid is introduced at the valve inlet, it is free to move into this intermediate chamber through a continuously open small offset passage extending through the diaphragm.
The intermediate chamber is then at a higher pressure than the outlet and the main diaphragm valve assembly is urged to its shut-off position due to the differential area caused by the main valve seat and the biasing force of the coil spring acting on the electromagnetic actuator armature.
When the windings of the electromagnetic coil are energized, the magnetic flux generated thereby moves the ferromagnetic actuator away from the diaphragm against the biasing force of the coil compression spring and fluid differential pressure force to unseat the pilot seal from the pilot passage in the diaphragm permitting flow through the pilot passage from the intermediate chamber to the outlet. This, of course, results in a reduction in pressure in the intermediate chamber and when the fluid pressure differential acting on the lower side of the diaphragm and the upper side of the diaphragm(intermediate chamber pressure) is sufficient, the diaphragm assembly, and hence the main valve will move upwardly away from the main valve seat permitting high pressure fluid to flow directly from the inlet to the outlet.
Because of the high initial current flow through the coil required to raise the actuator against the bias spring force and pressure differential force and cause opening movement of the main diaphragm valve assembly, the actuator will continue to move a considerable distance away from the diaphragm assembly until the magnetic actuation forces reach equilibrium with the spring force. This equilibrium point positions the actuator almost entirely within an associated guide bore in the coil assembly so that there is no modulation or restriction between the actuator seal and the pilot passage during this movement. The following diaphragm assembly then moves away from the main valve opening until it abuts a stop.
While the McCarty, et al. valve has been employed commercially solely as an on-off flow control valve as completely described in their patent, there have been unsuccessful experimental attempts to utilize the McCarty, et al. valve as a flow modulating valve. In these attempts, current flow through the coil was reduced after the initial high current flow to overcome the forces acting on the closed actuator, permitting the actuator to move downwardly as a result of the diminution in the flux field acting on the actuator. As the actuator and its pilot seal move toward the pilot passage, the differential pressure between the valve inlet and the intermediate chamber is reduced to achieve main flow modulation. The pressure drop across the pilot seal however, increases due to the flow restriction caused by the close proximity of the pilot seal to the pilot passage. The proximity of the pilot seal to the pilot passage is critical to where modulation of the main valve can be maintained without the pressure drop force across the pilot seal increasing to a valve wherein an inadvertent shut-off of the pilot passage occurs.
Because the main valve appeared to follow the actuator, the McCarty, et al. diaphragm valve assembly, at first blush, appeared capable of operating as a modulating valve with the appropriate reduction of coil current after opening. However, testing the McCarty, et al. valve in attempted modulating positions, particularly when a high differential pressure exists across the diaphragm, indicated the valve assembly to be very unstable not only under transient changes in inlet pressure, and transient current surges through the coil, but when tried under most controlled coil voltages and pressures.
I have found this instability to result from the close proximity of the actuator pilot seal from the pilot passage in the main diaphragm valve assembly when the actuator and valve are attempted to be put into modulating equilibrium position. In an exemplary modulating equilibrium position, the main valve assembly is open and the actuator seal is closely spaced to the diaphragm pilot passage so that its pilot seal is restricting flow through the pilot passage. The pilot actuator is stationary because the electromagnetic flux force is held constant for that position and is balanced by the force of the spring and the axial differential pressure acting thereon. At this time the diaphragm is in a stationary position because the differential pressures acting on the diaphragm's upper and lower surfaces produces a force balance.
The requirement for the seal being very close to the pilot opening in this balance position is due to the fact that the seal must be this close to sufficiently restrict pilot flow through the pilot passage to achieve the proper balance of forces acting on the main valve. The instability is also accentuated by the fact that there is a net downward differential pressure force acting on the actuator in this equilibrium position urging it to its closed position blocking flow through the pilot passage of the valve. This net differential pressure force is caused by the restriction established at the actuator seal area of the diaphragm pilot passage and acts across the area of the actuator seal immediately adjacent the pilot orifice to create a downward force on the actuator. In this nearly closed position of the actuator seal, any transient increase in inlet or intermediate chamber pressure increases the differential pressure and downward force acting on the actuator frequently causing inadvertent shut-off. The same result occurs from a transient drop in coil current.
While the McCarty, et al. valve is an excellent design for an on-off valve for which it is intended when originally designed, the tendency of the pilot valve to inadvertently close is unacceptable in a modulating valve since it drives the main valve to a closed position when it is not desired.
In my prior U.S. Pat. No. 4,863,098, another system for preventing inadvertent valve closure is disclosed. In that control valve an actuator pilot seal functions only to initiate opening and closing movement of the main valve and does not modulate pressure in the intermediate chamber, which is performed by a secondary valve modulating flow through an offset passage. Because of this the actuator, and particularly the actuator pilot seal, is by design positioned sufficiently far away from the central pilot passage in the control valve assembly so that when the secondary valve is modulating or restricting the offset passage, it is unaffected by pilot passage pressure or pressure differential forces and, hence there is no significant differential fluid pressure force acting on the actuator when the control valve is open. Since the differential pressure force acting on the actuator remains essentially zero during modulation, transient changes in intermediate chamber pressure create no imbalance of the forces acting on the actuator and hence no unwanted downward pilot actuator forces. The force balance and position of the control valve utilizing the secondary valve is inherently stable. That is, whenever transient conditions occur to upset the desired force balance across the control valve, the secondary valve either opens or restricts to change the intermediate chamber pressure. This causes movement of the control valve in a direction opposite to the disturbing transient so that the correct control valve force balance is always maintained.
Furthermore, the significant spacing of the actuator pilot seal from the pilot passage also reduces the likelihood of other factors causing inadvertent downward relative movement of the actuator including transient inlet fluid pressure rise and transient current dips in the coil.
According to another embodiment disclosed in my prior patent, the pivotal secondary valve is replaced with a valve disk fixed to the lower end of the actuator that actuates a poppet valve reciprocably mounted in a stepped bore in the valve assembly interconnecting the inlet chamber and the intermediate chamber. This embodiment operates in the same manner and has the same advantages of the pivotal secondary valve in the first embodiment except for the mechanical advantage achieved by the pivotal valve. In the second embodiment, however, no spring is required to bias the secondary poppet valve to its closed position since the valve is constructed to be flow responsive and is made of a light-weight plastic to achieve buoyancy toward the valve closed position.
Another problem in pilot operated flow modulating control valves is a tendency for the main valve member to chatter or vibrate when the valve is very close to seat when both opening and closing. This problem is also related to the well known "water hammer" effect in which a hydraulic shock wave is caused by the rapid opening and closing movement of control and shut-off valves in hydraulic systems.
It is a primary object of the present invention to ameliorate the problems noted above in pilot operated diaphragm control valves.